You can specify that you wish to use the Corax engine in 3 ways/levels:

1. Setting the search engine server-wide (for all databases):
   update your settings.json file as shown in this link:
   https://ravendb.net/docs/article-page/6.2/csharp/indexes/search-engine/corax#select-search-engine-server-wide
2. Setting the search engine per database:
   done via the Studio as shown in this link:
   https://ravendb.net/docs/article-page/6.2/csharp/indexes/search-engine/corax#select-search-engine-per-database
3. Setting the search engine per index:
   can be done via the client API code as shown in this link:
   https://ravendb.net/docs/article-page/6.2/csharp/indexes/search-engine/corax#select-index-search-engine-using-code